Learn Russian: Это очень интересное мероприятие, поэтому там много участников. - This is a very interesting event, so there are many participants there.

It comes immediately after the quantity word много (many). In Russian, when you talk about quantities using words like много, мало (few), or сколько (how many) with countable objects, the noun must be in the genitive plural form. Since участник is a masculine noun ending in a consonant, its genitive plural is formed by adding -ов.

Why does the adjective интересное end in -ое?

Adjectives must match the gender of the noun they describe. The word for event, мероприятие, ends in -е (specifically -ие), which means it is a neuter noun. Therefore, the adjective takes the neuter singular ending -ое.

I often confuse поэтому and потому что. Why is поэтому used in this sentence?

Поэтому translates to "therefore" or "so" and is used to introduce the result of something. Потому что means "because" and introduces a reason. Here, the first half of the sentence is the reason (the event is interesting) and the second half is the result (so there are many participants), which makes поэтому the correct choice.

At the beginning of the sentence, why do we use Это instead of matching the gender of the noun?

When you are introducing a noun by saying "This is a [noun]", you always use the fixed starting word Это, no matter the gender or number of the noun. You would only match the gender to a different word if you were saying "This event" as the subject, but here it acts as a general "This is..." statement.